The vertical optical coupling of AlGaN/GaN quantum well mid-infrared photodetector is enhanced by using two-dimensional gold grating structure, which is not sensitive to the polarization of incident light source. The electric field distribution and energy flow in the detector structure are calculated by the finite element method (FEM). It is found that the effective electric field intensity of quantum well with two-dimensional gold grating is two orders of magnitude higher than that of Si3N4 grating. It is mainly due to the surface plasmon polariton (SPP) excited at the interface of gold grating and AlGaN, which strongly changes the direction of light energy flow and electric field vibration. No matter how the light source polarization is, when the vertical incident electromagnetic wavelength is 4.7 μm, electric field intensity E is 1 V/m, the effective |Ez|2 in quantum well region reaches 0.7 (V/m)2, which provides a solution to realize the focal plane array of AlGaN/GaN quantum well mid-infrared photodetector.